U.S. patent number 5,632,921 [Application Number 08/463,217] was granted by the patent office on 1997-05-27 for cylindrical microwave heating applicator with only two modes.
This patent grant is currently assigned to The Rubbright Group, Inc.. Invention is credited to Charles R. Buffler, Per O. Risman.
United States Patent |
5,632,921 |
Risman , et al. |
May 27, 1997 |
**Please see images for:
( Certificate of Correction ) ** |
Cylindrical microwave heating applicator with only two modes
Abstract
A microwave applicator including a generally cylindrical
microwave containment chamber, a microwave energy source, and a
feed structure connecting the microwave energy source to the
containment chamber. The diameter of the containment chamber is
designed according to a process that need only take into account
supporting a microwave pattern having substantially only two
transverse magnetic modes, each with a characteristic guide
wavelength, where the guide wavelength of one mode is substantially
equal to twice the guide wavelength of the other mode.
Additionally, the interior diameter can be sized to minimize the
index subscript numbers of the transverse magnetic modes. The feed
structure of the applicator includes at least two feed apertures
spaced physically apart around the cylindrical axis of the
applicator by a physical angle equal to an electrical phase shift
angle of the microwaves introduced through the respective
apertures, which in a preferred embodiment is 90 degrees.
Inventors: |
Risman; Per O. (Harryda,
SE), Buffler; Charles R. (Marlborough, NH) |
Assignee: |
The Rubbright Group, Inc.
(Eagan, MN)
|
Family
ID: |
23839318 |
Appl.
No.: |
08/463,217 |
Filed: |
June 5, 1995 |
Current U.S.
Class: |
219/750; 219/697;
219/756; 219/746 |
Current CPC
Class: |
H05B
6/707 (20130101); H05B 6/6402 (20130101) |
Current International
Class: |
H05B
6/70 (20060101); H05B 6/80 (20060101); H05B
006/72 () |
Field of
Search: |
;219/750,746,756,762,690,695,696,697 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
One page, two photographs of a grain dryer believed to be useful as
a microwave grain dryer in the practice of the process of U.S.
patent 4,728,522 and the apparatus of U.S. patent 4,785,726, dated
before Jun. 5, 1995..
|
Primary Examiner: Leung; Philip H.
Attorney, Agent or Firm: Faegre & Benson
Claims
What is claimed is:
1. A microwave applicator for heating a load, the applicator
comprising:
a) a microwave containment chamber to contain microwaves, the
chamber having a top wall, a bottom wall and a generally
cylindrical side wall, the side wall connected to the top wall, the
containment chamber having an interior diameter;
b) a microwave energy source for generating microwaves at a
predetermined frequency; and
c) a feed structure connected between the microwave energy source
and the containment chamber for coupling microwaves from the energy
source to the containment chamber
wherein the diameter of the containment chamber is sized to support
a microwave field having only first and second transverse magnetic
modes, each mode having a respective guide wavelength, and wherein
the guide wavelength of the first mode is substantially equal to
twice the guide wavelength of the second mode.
2. The microwave applicator of claim 1, wherein the cylindrical
side wall has a cylindrical longitudinal axis and a generally
circular cross-section normal to the longitudinal axis.
3. The microwave applicator of claim 1, wherein the cylindrical
side wall has a longitudinal axis and a polygonal cross-section
having at least five sides normal to the axis.
4. The microwave applicator of claim 1, wherein the interior
diameter of the chamber is selected to minimize the index numbers
of the first and second transverse modes.
5. The microwave applicator of claim 1, wherein the interior
diameter of the chamber is sized to produce a TM.sub.02 mode as the
first mode and a TM.sub.11 mode as the second mode at the
predetermined frequency.
6. The microwave applicator of claim 1, wherein the predetermined
frequency is substantially equal to 2450 MHz.
7. The microwave applicator of claim 6, wherein the interior
diameter is about 9.17 inches.
8. The microwave applicator of claim 6, wherein the cylindrical
side wall has an interior height of about 6.28 inches.
9. The microwave applicator of claim 1, further comprising a
microwave transparent shelf for supporting the load located inside
the containment chamber and generally parallel to the top wall.
10. The microwave applicator of claim 9, wherein the shelf is
placed at a distance from the top wall such that the load rests at
a distance from the top wall substantially equal to an integer
multiple of the guide wavelength of the second mode.
11. The microwave applicator of claim 9, wherein the shelf is made
of borosilicate glass.
12. The microwave applicator of claim 9, wherein the shelf is made
of glass ceramic.
13. The microwave applicator of claim 1, wherein the cylindrical
side wall has an opening and the applicator further comprises a
movable door congruent to and selectively closing the opening in
the cylindrical side wall.
14. The microwave applicator of claim 13, further comprising a
slidable drawer attached to the door, the drawer for inserting the
load into the containment chamber.
15. The microwave applicator of claim 1, wherein the bottom wall is
shaped to have a surface of revolution about a cylindrical
longitudinal axis of the containment chamber.
16. The microwave applicator of claim 15, wherein the interior
diameter is selected to support a microwave field in the chamber
substantially lacking a transverse electric field component with
respect to the cylindrical longitudinal axis.
17. The microwave applicator of claim 16 wherein the microwave
field in the chamber lacks a transverse electric field component in
the circumferential direction.
18. The microwave applicator of claim 1, wherein the feed structure
comprises a first waveguide feed and a second waveguide feed each
coupled to the containment chamber and located apart at the points
of entry into the containment chamber substantially at a physical
angle with respect to each other equal to an electrical phase angle
difference between the microwaves at the respective points of entry
into the containment chamber.
19. The microwave applicator of claim 18, wherein physical and
electrical phase shift angles between the waveguide feeds are each
equal to ninety degrees.
20. The microwave applicator of claim 18, wherein each waveguide
feed further comprises a feed aperture located on the top wall.
21. The microwave applicator of claim 18, wherein the feed
structure includes a main waveguide coupled to the first and the
second waveguide feeds and a pair of feed apertures located in the
top wall.
22. The microwave applicator of claim 18, wherein the first
waveguide feed connects into a first feed aperture on the side
wall, and the second waveguide feed connects to a second feed
aperture located in the side wall at a physical angle of ninety
degrees from the first feed aperture.
23. The microwave applicator of claim 22, wherein the feed
structure further includes a phase shift structure to phase shift
the microwaves entering the chamber from the second waveguide feed
ninety electrical degrees with respect to the microwaves entering
the chamber from the first waveguide feed.
24. The microwave applicator of claim 23, wherein the phase shift
structure includes a junction connecting the first and the second
waveguide feeds and a different length between the first and the
second waveguide feeds such that the second waveguide feed phase
shifts the microwaves entering the chamber from the second
waveguide feed ninety degrees from the microwaves entering the
chamber from the first waveguide feed.
25. The microwave applicator of claim 23, wherein the phase shift
structure includes a dielectric phase shifter.
26. The microwave applicator of claim 23, wherein the phase shift
structure includes a ferrite phase shifter.
27. A microwave containment chamber for a microwave applicator for
heating a load, the chamber comprising:
a) a top wall;
b) a bottom wall; and
c) a generally cylindrical side wall connected to the top wall, and
sealed sufficiently to contain microwaves, the cylindrical side
wall having an interior diameter;
wherein the interior diameter of the cylindrical side wall is sized
to support a microwave field having first and second transverse
magnetic modes, and characterized by the absence of transverse
electric modes, each transverse magnetic mode having a respective
guide wavelength, and wherein the guide wavelength of the first
transverse magnetic mode is substantially twice the guide
wavelength of the second transverse magnetic mode.
28. A microwave applicator for heating a load, the applicator
comprising:
a) a generally cylindrical microwave containment chamber having a
continuous side wall, a generally planar top wall, and a bottom
wall, the side wall connecting the top wall and the bottom wall,
the top, bottom, and side walls sealed and secured together to
contain microwave energy, the containment chamber having an
interior diameter;
b) a microwave energy source for generating microwave energy at a
frequency substantially equal to 2450 megahertz;
c) a feed structure connected between the microwave energy source
and the containment chamber for coupling the microwave energy from
the energy source to the containment chamber;
wherein the interior diameter of the containment chamber is
sufficiently close to 9.17 inches to support a microwave field
having only a TM.sub.02 first transverse magnetic mode and a
TM.sub.11 second transverse magnetic mode, each mode having a
respective guide wavelength, and such that the guide wavelength of
the first mode is substantially equal to two times the guide
wavelength of the second mode.
29. A method of manufacturing a microwave containment chamber for a
microwave applicator, comprising the steps of:
a) forming a generally cylindrical side wall of a conductive
material sufficient to contain microwaves, the side wall having two
open areas at longitudinal ends thereof and having an interior
diameter, wherein the step of forming the side wall includes the
step of selecting the interior diameter of the side wall such that
introduction of microwaves at a predetermined frequency into the
circularly cylindrical side wall produces a microwave field having
first and second transverse magnetic modes each having a respective
guide wavelength, the first mode having two times the guide
wavelength of the second mode such that the combination of the
first and second modes provides a substantially even heating
pattern;
b) providing a bottom wall and a top wall of a conductive material
sufficient to contain microwaves, the bottom and top walls having
dimensions at least sufficient to close the open areas of the side
wall;
c) connecting the top wall to one of the open areas of the side
wall, such that the top wall closes the one open area and
connecting the bottom wall to the other open area of the side wall,
such that the bottom wall closes the other open area, the top,
bottom and side walls together forming a closed-ended generally
cylindrical chamber.
30. The method of manufacture of claim 29, wherein the step of
selecting the interior diameter of the side wall further comprises
minimizing the index numbers of the first and second modes.
31. The method of manufacture of claim 29, wherein the step of
selecting the interior diameter of the side wall includes sizing
the interior diameter to produces a microwave field having a
TM.sub.02 first transverse magnetic mode and a TM.sub.11 second
transverse magnetic mode.
32. The method of manufacture of claim 29, wherein the step a)
includes sizing the interior diameter to be substantially equal to
9.17 inches.
33. The method of manufacture of claim 29, wherein the step of
forming the side wall includes providing a side wall having a
generally circular normal axis cross sectional profile.
34. The method of manufacture of claim 29, wherein the step of
forming the side wall includes providing a cylindrical side wall
having a normal axis cross-sectional profile generally shaped as a
higher-order polygon.
35. The method of manufacture of claim 29, wherein the step of
providing the bottom wall further includes the step of shaping the
bottom wall to have a surface of revolution about a longitudinal
axis of the containment chamber.
36. The method of manufacture of claim 29, further including the
steps of providing a microwave transparent shelf for supporting the
load and of positioning the shelf inside the containment chamber
and generally parallel to the top wall.
37. The method of manufacture of claim 36, wherein the step of
positioning the shelf includes locating the shelf to support the
load at a distance from the top wall substantially equal to an
integer multiple of the guide wavelength of the second mode.
38. The method of manufacture of claim 29, further including the
steps of forming an opening on the side wall, providing a door
sized to cover the opening, and attaching the door to the microwave
applicator such that the door selectively closes the opening.
39. The method of manufacture of claim 29, further including the
steps of:
d) forming an opening on the side wall;
e) providing a door connected to a drawer, including sizing the
door to cover the opening and sizing the drawer to fit through the
opening; and
f) slidably receiving the drawer in the opening such that the door
selectively closes the opening.
Description
FIELD OF THE INVENTION
The present invention is directed to a microwave applicator. More
specifically, the invention is directed to a high efficiency
generally cylindrical microwave applicator having a specially sized
microwave containment chamber with low leakage and a feed system
which provides a rotating field without moving parts for even
heating of a load.
BACKGROUND OF THE INVENTION
As is well-known, electromagnetic waves can transport and deliver
energy to an object or load. Microwave applicators using
electromagnetic waves in a frequency range of 300 MHz to 300 GHz
generally include a microwave energy source, a microwave
containment chamber, and a microwave feed structure coupling the
energy source to the microwave containment chamber.
A preferred microwave energy source for the present invention is a
magnetron operating at 2450 MHz, although it is to be understood
that since 915 MHz is an approved microwave cooking and heating
frequency, the present invention is adaptable to operation at 915
MHz, and any other microwave frequency desired, according to the
teachings hereof.
The volumetric space within a microwave containment chamber is a
cavity in which the load (the object or substance to be heated) is
placed.
One of the most significant problems with prior art microwave
applicators is uneven temperature distribution in the load. Uneven
heating is mainly due to three causes: mode-related hot and cold
spots, edge overheating, and underside underheating.
Each mode has a respective vertical guide wavelength
.lambda..sub.g. When modes in a system can be excited so that the
modes do not couple to each other even if the system is lossy, the
modes are called orthogonal modes.
In the prior art, hot and cold spots occurred because of the uneven
energy distribution particular to the modes in the cavity of the
applicator. The electric and magnetic field configuration of a mode
is dependent on the operating frequency and the dimensions of the
cavity.
There are two distinct classes of modes, transverse magnetic (TM)
modes and transverse electric (TE) modes. TE modes have no electric
or E field component in a direction of propagation, while TM modes
have no magnetic or H field component in the direction of
propagation.
TE and TM modes are labelled as TE.sub.mn and TM.sub.mn. For a
rectangular waveguide, the subscripts indicate the number of
half-period variations of a mainly transverse field vector along
paths parallel to a wide wall (m) and a narrow wall (n). In a
rectangular coordinate system, the m and n subscripts
conventionally refer to the x and y axes, with propagation
occurring along the z axis.
In a cylindrical cavity it is convenient to use a polar coordinate
system. In the present invention, the direction of propagation is
along a z axis parallel to the longitudinal cylindrical axis of the
cylindrical cavity. In a circular cross-section waveguide or
cavity, i.e., one having a generally circular wall concentric to
the direction of propagation of microwave energy in the waveguide
or cavity, the subscript or index m indicates the number of
full-period variations of a transverse field vector along a
circular path concentric with the wall. Subscript or index n
indicates the number of reversals plus one of the same vector along
a radial path in the cavity.
The traditional solutions to avoid mode-related hot and cold spots
were either to use a mechanical device (e.g., a turntable) to move
the load in relation to the cavity during heating or to use a "mode
stirrer" to continually alter the mode patterns within the cavity.
Mode stirrers are typically fan-shaped mechanically rotating
structures with metal blades placed either inside the cavity or in
a separate open feedbox adjacent the cavity. Some designs have
attempted to reduce hot and cold spots by using devices such as
multiple feed arrangements or rotating antennae.
There continues to exist a need for an efficient microwave
applicator that offers convenient and reliable time-averaged
uniformity of microwave heating.
Edge overheating (hot spots on the edges of the load) occurs due to
the direct coupling of an E field component parallel to an edge of
the load, and becomes more significant when the load has a high
permittivity.
In most microwave ovens, the loads are generally dielectrics, such
as food, with a rather high relative permittivity. The microwave
modes interact with the high .epsilon. load to transfer energy into
the load .epsilon..
It is important to understand that the H field intensity in the
load and the heating pattern are directly related. Maxwell's
equations reveal that energy absorption of the load is generally
through the electric E field. Prior art applicators attempt to
maximize E and H field intensity to maximize energy transfer and
minimize cooking time. However, in so doing, the prior art
applicators increase edge overheating, and the possibility of
microwave leakage.
Another microwave heating problem is low or insufficient
"underside" heating of a flat load. Since not much power penetrates
through a flat load, the underside of a flat horizontal load is
usually poorly and unevenly heated. Absent a microwave feed below
the load, "underside" heating requires the load to not extend over
the whole cross section of the cavity.
SUMMARY
The present invention is a microwave applicator for evenly heating
a relatively flat load, substantially eliminating uneven heating
evidenced by hot and cold spots and edge overheating. The
applicator uses modes in the cavity that offer high-efficiency by
being frequency broadband, maximizing cooking energy in the load,
minimizing microwave leakage, and at the same time both reducing
load edge overheating and increasing load underside heating. The
applicator includes a feed structure that works in conjunction with
the cavity modes to evenly distribute energy to the load without
any moving parts.
The applicator includes a microwave containment chamber, a
microwave energy source, and a feed structure connecting the
microwave energy source to the containment chamber. The applicator
can also include electronic controls to control the microwave
energy source.
The microwave energy source is preferably a magnetron generating
microwaves at a predetermined frequency (2450 or 915 Mhz in
alternative preferred embodiments). The feed structure guides the
microwaves from the energy source to the containment chamber.
The containment chamber is formed of microwave reflective material
and is designed to prevent leakage of microwave energy to the
environment outside the containment chamber. The chamber has a top
wall, a bottom wall and a side wall. The side wall (which is
preferably cylindrical) extends between the top and bottom walls,
surrounding (and defining) the cavity and is aligned with a
longitudinal axis. In contrast to a conventional microwave oven
cavity, the containment chamber preferably has a generally circular
cross-section normal to the longitudinal axis, however it is to be
understood that the cross-section can be shaped as another closed
plane figure, such as a polygon having at least five sides,
provided that the cavity cross section approximates a circle. The
top and bottom walls are preferably characterized by a surface of
revolution about the longitudinal axis, and are preferably
planar.
The containment chamber has an interior diameter corresponding to
an actual or average diameter of the cross section of the chamber
and an interior height equal to a distance between the top and
bottom walls. In the practice of the present invention, the
interior diameter is designed according to a process which takes
into account only transverse magnetic modes to support a desired
microwave field in the chamber. While the design criteria involve
only TM or transverse magnetic modes, it has been observed that the
actual modes present in the cavity as a result of using this design
technique are of the more complex hybrid mode types which means
they are composed of simultaneous TE and TM modes with the same or
similar .lambda..sub.g.
Nevertheless, in the practice of the present invention, it has been
found adequate to use the techniques presented herein to design a
cavity capable of supporting a microwave field having only two
transverse magnetic modes, with each having a characteristic guide
wavelength, where the guide wavelength of one mode is substantially
equal to twice the guide wavelength of the other or second mode.
Preferably, the interior diameter is sized or chosen to minimize
the index subscript numbers of the TM modes used in the design of
the chamber.
In a first preferred embodiment, the interior diameter of the
chamber is designed to produce a TM.sub.02 mode as the first mode
and a TM.sub.11 mode as the second mode. At a predetermined
frequency of 2450 Mz, the interior diameter of this embodiment is
preferably about 9.17 inches (233 mm) and the load height (h) to
the top of the load is preferably about 6.28 inches (160 mm).
The microwave applicator of the present invention also preferably
includes a shelf (made of borosilicate glass, glass ceramic, or
other similar microwave transparent materials) for supporting the
load. The shelf is located inside the containment chamber and is
generally perpendicular to the longitudinal axis. The shelf is
desirably placed at a distance from the top wall such that the load
rests at a distance from the top wall substantially equal to an
integer multiple of the guide wavelength of the second (shorter)
mode.
The side wall of the microwave applicator preferably has a
load-insertion opening and a movable door to selectively close the
opening. In one embodiment, a slidable drawer can be attached to
the door, with the drawer adapted for inserting the load into the
containment chamber. When a drawer is used, the shelf is preferably
part of or carried by the drawer.
The feed structure of the microwave applicator of the present
invention includes a main waveguide, one or more junctions, and a
plurality of waveguide feeds. The waveguide feeds are short
waveguides each attached on one end to a feed aperture on the
containment chamber and on the other end to the main waveguide at a
junction (which may be common to both waveguide feeds or a separate
junction for each). The feed apertures can be located on the top
wall or on an upper portion of the side wall. The feed apertures or
ports are to be located at a physical angle (with respect to the
longitudinal axis) that is equal to an electrical phase angle by
which the microwaves are displaced as they enter the cavity. In a
preferred embodiment the first waveguide feed connects into a first
feed aperture and the second waveguide feed connects to a second
feed aperture in geometric quadrature, that is, the second feed
aperture is physically located ninety degrees apart from the first
feed aperture, as measured in a plane normal to the longitudinal
axis.
Additionally in this embodiment the feed structure includes a phase
shift structure to shift the electrical phase of microwaves
entering the chamber from the first waveguide feed to be ninety
degrees apart from the electrical phase of microwaves entering the
chamber from the second waveguide feed. In this way, two streams of
microwave energy are provided, with each stream separated both
ninety degrees physically and ninety degrees out of phase
electrically from the other as they enter the containment
chamber.
The phase shift structure can be any conventional means of
achieving ninety degrees phase shift between the first and second
waveguide feeds. The length of the waveguide feeds from their
junction (or the location of respective separate junctions) with
the main waveguide to the respective feed apertures can be
different such that the second waveguide feed phase shifts the
microwaves ninety degrees with respect to the microwaves entering
the chamber from the first waveguide feed. Alternatively, the phase
shift structure can use a dielectric phase shifter or a ferrite
phase shifter, or other phase shifters as are well known in the
art. The combined effect of the geometric quadrature and the ninety
degree phase shift produces a rotating microwave pattern in the
cavity, thus producing more even heating in the absence of
physically rotating or moving parts in the feed structure.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of a microwave applicator in
accordance with the present invention.
FIG. 2 is a perspective view of the drawer from the microwave
applicator of FIG. 1.
FIG. 3 is a side view of the drawer of FIG. 2.
FIG. 4 is an exploded perspective view of another embodiment of a
microwave applicator in accordance with the present invention.
FIG. 5 is a graph representing the relation between the guide
wavelength and the waveguide diameter for several modes for a 2450
MHz microwave field.
FIG. 6 is a perspective view of a first embodiment of a feed
structure in accordance with the present invention.
FIG. 7 is a perspective view of a second embodiment of a feed
structure in accordance with the present invention.
FIG. 8 is a perspective view of a third embodiment of a feed
structure in accordance with the present invention.
FIG. 9 is a simplified view of the top of a microwave containment
chamber showing the axes of entry of a pair of microwave feeds
illustrating certain aspects of the present invention.
FIG. 10 is a simplified side view in section of the microwave
containment chamber of FIG. 9 showing a shelf and load in
phantom.
FIG. 11 is a fragmentary enlarged perspective view of a side wall
iris feed aperture useful in the practice of the present
invention.
FIG. 12 is a fragmentary enlarged perspective view of a top wall
iris feed aperture with a portion of the waveguide feed cut
away.
FIG. 13 is a simplified top and side view of a cavity useful in the
practice of the present invention showing a TM.sub.11 mode.
FIG. 14 is a simplified top and side view of the cavity of FIG. 13
showing a TM.sub.02 mode.
FIG. 15 is a simplified top view of a containment chamber and
waveguide feeds showing a TM.sub.11 mode of the cavity field at a
first electrical phase condition of the present invention.
FIG. 16 is a simplified top view similar to that shown in FIG. 15
but with the TM.sub.11 mode of the cavity field shown at a second
electrical phase condition advanced ninety electrical degrees
therefrom.
FIG. 17 is a view similar to FIG. 15, except advanced ninety
electrical degrees from FIG. 16, thus being 180 electrical degrees
advanced from FIG. 15.
FIG. 18 is a view similar to FIG. 17, except advanced an additional
ninety electrical degrees, thus being 270 electrical degrees
advanced from FIG. 15.
DETAILED DESCRIPTION
The present invention is a microwave applicator for providing
efficient and even heating to a load by substantially eliminating
hot spots and cold spots. In addition, the applicator of the
present invention uses cavity modes which substantially eliminate
edge overheating, reduce microwave energy leakage, and are highly
efficient.
FIG. 1 illustrates a microwave applicator 10 in accordance with the
present invention. The applicator 10 includes a microwave
containment chamber 20, an energy source 50 and a feed structure 60
coupling the energy source 50 to the containment chamber 20. The
energy source 50 is a magnetron or other source designed to
producing microwaves at a predetermined frequency, most commonly at
either 2450 or 915 MHz. Electronic controls 90 allow a user to
control both the time during which the magnetron is activated and
the power setting of the magnetron. Different power settings are
usually achieved by periodic on/off duty cycles of the
magnetron.
Referring now also to FIGS. 9 and 10, the microwave containment
chamber 20 is a container or enclosure made of microwave reflective
material such as metal enclosing a cavity in which a load 80 (the
substance to be heated) is placed. A typical preferred load 80 for
the microwave applicator of the present invention (as shown in FIG.
10) is characteristically flat and horizontally extended, such as a
pizza or sandwich. It is to be understood that non-flat loads can
also be heated with the applicator of the present invention, but
that the benefits of the present invention are best achieved with
relatively flat loads. The chamber 20 has a cylindrical
longitudinal axis z, a generally cylindrical side wall 22, a top
wall 24, and a bottom wall 26.
Microwave applicator 10 also includes a microwave transparent shelf
12 for supporting the load 80. The shelf 12 is located inside the
containment chamber 20 and is generally parallel to the top wall
24. In a preferred embodiment, the shelf 12 is made of borosilicate
glass, glass ceramic or other microwave transparent materials.
The microwave containment chamber 20 has an interior diameter D, an
interior height H, and a load height h. The diameter D is the
diameter of a cross-section perpendicular to the longitudinal axis
z of the cavity, as may best be seen in FIG. 10. The height H is
the distance between the top wall 24 and the bottom wall 26, and is
to be understood that H is the "effective" height in the event the
top or bottom wall is non-planar. The load height h is the distance
from the top wall 24 to the load 80.
Referring now again to FIGS. 1-4, the side wall 22 and the chamber
20 form a right circular cylinder. In other embodiments, the side
wall 24 can have a cross section normal to the longitudinal axis z
shaped as other closed plane curves or as a higher-order polygon,
i.e., a polygon having five or more sides. It is to be understood
that such a polygonal embodiment must approximate a circle to some
degree to obtain certain benefits of the present invention.
Furthermore, it is also to be understood that if a polygon is
chosen for the cross section of applicator, a regular polygon
(i.e., one with equal sides) is preferred, although it is possible
to obtain certain benefits of the present invention with an
asymmetrical polygon as well.
Containment chamber 20 has a load-insertion opening 28 in the side
wall 22. The opening may be generally quadrilateral or rectangular
and is generally normal to the longitudinal axis z. A movable door
30 is congruent to and selectively closes and seals the opening 28
against microwave leakage. In one embodiment, a slidable drawer 32
for inserting the load 80 into the containment chamber 20 may be
attached to the door 30, or may be separately located in chamber
20. The shelf 12 may be located on the drawer 32. Other embodiments
can include different door elements, for example, the embodiment
shown in FIG. 4 has a planar door 30' secured to a lower housing 36
by a piano hinge 40. The shelf can be a part of the drawer itself
or can rest in a selected position in the cavity.
In the practice of the present invention, the interior diameter D
of chamber 20 is designed using a technique intended to result in a
microwave field in the chamber 20 having only transverse magnetic
modes present in any plane normal to the longitudinal axis z. More
particularly, containment chamber 20 is sized according to a design
which need only take into account supporting a microwave field
having only a first TM mode and a second TM mode, where the first
TM mode has a guide wavelength that is substantially equal to twice
the guide wavelength of the second TM mode. Containment chamber 20
is also preferably sized to tend to minimize the index numbers of
the first and the second transverse magnetic modes. Again, it is to
be stressed that although the design process is directed to
producing only TM modes, the actual field in the cavity of chamber
20 may actually have hybrid modes present, while still achieving
the benefits of the present invention.
In one embodiment, the diameter D of containment chamber 20 is
substantially equal to 9.17 inches (233 mm). The interior height H
of containment chamber 20 is approximately 7.00 inches (178 mm). In
this embodiment, the interior diameter D of the chamber 20 is sized
to produce a TM.sub.02 mode as the first mode and a TM.sub.11 mode
as the second mode at the predetermined frequency of 2450 MHz. The
first (TM.sub.02) mode has a guide wavelength .lambda..sub.g1 that
is substantially equal to twice the guide wavelength
.lambda..sub.g2 of the second (TM.sub.11) mode. The modes have
favorable and complementary field patterns.
In containment chamber 20, the shelf 12 is placed to provide a
distance h of 6.28 inches (160 mm) from the top wall 24 to the load
80. It has been found preferable for the load 80 to be located at a
distance h between the top wall 24 and the top of the load 80 (for
a flat, horizontally extending load) substantially equal to an
integer multiple of the guide wavelength of the second TM mode.
Accordingly, other embodiments can place the shelf at different
locations (or at an "average" fixed location) to accommodate loads
of assorted thicknesses, keeping in mind the desired integer
multiple relationship.
FIG. 5 illustrates the relationship between the guide wavelength
.lambda..sub.g of different modes and the diameter D of a generally
circular waveguide. In FIG. 5, the guide wavelength is shown (in
inches) along the ordinate or vertical axis and the diameter (in
inches) is shown along the abscissa or horizontal axis. The
TM.sub.2 mode is represented by the curve identified by inverted
triangles, while the TM.sub.2 mode is identified by "x"s. The
upright triangles represent both TE.sub.01 and TM.sub.11 modes,
while the diamonds represent the TE.sub.21 mode and the squares
represent the TE.sub.11 mode. The "+"s (between the diamonds and
squares) represent the TM.sub.01 mode. Given the design requirement
of the present invention that .lambda..sub.g1 =2.lambda..sub.g2, it
can be seen that only certain diameter D sizes and first and second
TM mode pairs can be selected. The diameter and height information
with matching modes is also presented in tabular form in Table
1.
TABLE 1 ______________________________________ SEC- CAVITY FIRST
OND DIAMETER LOAD INTERIOR MODE MODE (D) HEIGHT(h) HEIGHT(H) (TM)
(TM) (inches) (inches) (inches)
______________________________________ 21 01 8.85 5.30 6.2 11 01
6.45 5.88 6.6 21 11 8.43 6.72 7.5 02 21 8.66 11.64 12.4 02 11 9.17
6.28 7.0 02 01 9.53 5.23 6.0
______________________________________
As may be seen, there are other embodiments having different
diameters and heights which support other first and second TM
modes. For all embodiments, the guide wavelength of the first TM
mode is substantially equal to twice the guide wavelength of the
second TM mode.
Use of the methodology of the present invention to size the cavity
of the containment chamber increases cooking efficiency and reduces
edge overheating, because of certain benefits of TM modes present,
whether in "pure" form or in a hybrid form.
TE modes have impedances higher than the free space impedance,
.eta..sub.0, whereas TM mode impedances are lower than .eta..sub.0.
Since wave reflection at a boundary becomes zero when there is
impedance equality across it, TM modes are more favorable for
heating purposes, being better suited to match the impedances of
common loads, such as food items. Strong standing waves are not
required to be built up and the determination of the cavity height
and coupling factor for the containment chamber to become efficient
at resonance is not as critical as with TE modes. Conditions for
reflectionless transmission into a relatively thick load that
covers substantially the whole horizontal cross section of the
applicator can be established. Reflectionless transmission is
highly desirable, since energy reflected back toward the magnetron
reduces the efficiency of the applicator.
By sizing the containment chamber 20 to produce only TM modes, the
microwave applicator 10 is designed to avoid high horizontal E
field components, particularly near the edge regions of the load
80; it being understood that the modes present in the cavity,
whether TM or hybrid, have this lack of an E field component. Edge
overheating is avoided by designing the microwave field pattern to
eliminate (or minimize) any E field component parallel to the edge
of the load 80. This condition is achieved when the missing E field
component is circumferentially directed, accomplished by selecting
a "dominant" or strongly coupled mode having an initial index of
zero, e.g., TM.sub.02. An additional benefit in this case is that
leakage is reduced since any existing E fields are perpendicular to
the door opening 28. Using a TM.sub.02 mode alone would result in
unacceptable "cold" spots in the center and in a concentric ring or
annulus of the heating pattern in the cavity. To correct this,
another mode having a "hot" spot in the center of the cavity is
selected for use along with the TM.sub.02 mode. Using a TM.sub.11
mode will eliminate the "cold" spot in the resulting heating
pattern; and, using quadrature feed, the TM.sub.11 mode is rotated,
eliminating azimuthally displaced "hot" and "cold" spots associated
with the heating pattern resulting from a simple TM.sub.11 mode by
averaging or integrating the pattern circumferentially, as will be
described in more detail hereinafter.
FIG. 4 illustrates an exploded view of an alternative embodiment of
a microwave applicator 20' having a top wall 24', a cylindrical
side wall 22' and a bottom wall 26'. In the Figures, corresponding
structures are labelled with the same or primed (apostrophized)
reference numbers. In this embodiment, a rectangular lower housing
36 is provided, carrying shelf 12' and door 30' which is secured to
housing 36 by the piano hinge 40. It has been found that a
relatively short (i.e., less than about 15% of h) rectangular cross
section lower housing 36 does not significantly adversely affect
performance of the present invention in this embodiment. It may be
noted that the dimension H is made up of the height 40 of
cylindrical wall 22' plus the height 44 of lower housing 36. Such
an approach will simplify the design of the region containing the
load, especially the closure or door 30'.
Referring now to FIGS. 6, 7 and 8, an overall feed structure 160
includes a main waveguide 161, a first waveguide feed 162 extending
from the main waveguide 161 at a junction 163, and a second
waveguide feed 164 bifurcating from the main waveguide 161 and the
first waveguide feed 162 at the junction 163. In this version, the
main waveguide 161 is generally parallel to a top surface of top
wall 124 and may extend radially away from the containment chamber
120 as shown in FIG. 6, or it may extend along the cylindrical side
wall of the chamber, as shown in FIG. 1 in phantom. As shown in
FIG. 6, the first waveguide feed 162 extends longitudinally from
the main waveguide 161 across the top surface of top wall 124; it
is to be understood however that the main waveguide 161 (and
waveguide feeds 162, 164) can be positioned as desired with respect
to the chamber 120, provided that the feed apertures are properly
positioned with respect to the chamber 120. In this embodiment, the
second waveguide feed 164 extends perpendicularly from the first
waveguide feed 162 across the top surface of the top wall 124, with
an included angle 190 of ninety degrees.
The first and second waveguide feeds 162 and 164 are coupled to
containment chamber 120 through feed apertures of the type shown in
FIG. 12 as top feed aperture or iris 168 on the top surface of the
containment chamber 120. The first feed aperture associated with
the first microwave feed 162 is located ninety degrees (indicated
by angle 190, and axes 192, 194) from the second feed aperture
associated with the second microwave feed 164. This ninety-degree
displacement feed aperture arrangement is called geometric
quadrature. The axes 92, 94 of the feed apertures may be seen most
clearly in FIG. 9.
It is to be understood that the overall feed structure 160 also
includes a phase shift structure to phase shift the microwaves
entering the chamber from the second waveguide feed 164 ninety
degrees with respect to the microwaves entering the chamber from
the first waveguide feed 162. In feed structure 160, the phase
shift structure includes the junction 163, the first waveguide feed
162, and the second waveguide feed 164, with the length of each of
the waveguide feeds 162 and 164 from the junction 163 to the
respective feed apertures 166 and 168 sized such that the second
waveguide feed 164 phase shifts the microwaves ninety degrees
electrically with respect to the microwaves entering the chamber
120 from the first waveguide 162. In this way, the two waveguide
feeds 162 and 164 couple microwaves into the containment chamber
120 displaced ninety degrees from each other both physically and
electrically. Because of the vectorial addition property of
orthogonal modes, the resulting linearly polarized mode is
continuously rotated, as will be described in more detail with
respect to FIGS. 15-18.
FIG. 7 illustrates a second embodiment of a feed structure 260.
Feed structure 260 includes a main waveguide 261 having a junction
263 bifurcating into a first waveguide feed 262 positioned along an
axis 292 and a second waveguide feed 264 positioned along an axis
294. The first and second waveguide feeds 262 and 264 may, but do
not necessarily, extend generally parallel to the top wall 224. The
first and the second waveguide feeds 262, 264 are connected to feed
apertures 266, 268 respectively, which are placed on the top wall
224 in geometric quadrature with respect to each other, indicated
by the right angle 290 between axes 292 and 294 (with each
preferably having an aperture corresponding to iris 168 of FIG. 12
to couple energy to chamber 220). In addition, the first and second
waveguide feeds 262 and 264 are sized so that the microwaves from
the second waveguide feed 264 are ninety degrees out of phase
electrically with respect to the microwaves entering chamber 220
from the first waveguide feed 262.
FIG. 8 illustrates a third embodiment of a feed structure 360.
Overall feed structure 360 has a main waveguide 361, a junction
363, a first waveguide feed 362, and a second waveguide feed 364.
The first and second waveguide feeds each couple respectively to
first and second feed apertures 366 and 368, located in geometric
quadrature (i.e., ninety degrees mechanically or geometrically
apart, indicated by angle 390 between axes 392 and 394) on side
wall 322, with the details of each feed aperture matching that of
the iris 368 of FIG. 11.
The main waveguide 361 is generally perpendicular to the
longitudinal axis z, projecting radially from side wall 322 of
containment chamber 320. At junction 363, the first waveguide feed
362 extends radially inwardly from the main waveguide 361. The
second waveguide feed 364 extends from the main waveguide 361 and
connects to the second feed aperture 368.
The first and second waveguide feeds 362 and 364 are sufficiently
different in length so that the microwaves from the second
waveguide feed 364 are ninety electrical degrees out of phase with
respect to the microwaves entering chamber 320 from the first
waveguide feed 362.
Other embodiments of the feed structure (not shown) may be used
which have feed apertures in quadrature, for example, phase shifter
structures including a dielectric phase shifter or a ferrite phase
shifter.
It is to be understood that the apertures for coupling microwave
energy into the containment chamber from the respective microwave
feeds may take other, well-known forms (not shown, for example, a
probe projecting into the cavity), alternative to those shown in
FIGS. 11 and 12.
Referring now to FIGS. 13 and 14, a top view 400 and a side view
402 of a cavity containing a TM.sub.11 mode may be seen with field
lines illustrated graphically in a greatly simplified fashion, with
top views illustrating magnetic field lines and side views
illustrating electric field lines. Similarly, referring to FIG. 14,
a top view 404 and a side view 406 of a TM.sub.02 mode may be
seen.
Referring now to FIGS. 15 and 16, the operation of the rotating
field is illustrated in top views 408 and 410 which are to be
understood to be representations of the TM.sub.11 mode at different
times, with the different times corresponding to a ninety
electrical degree phase shift at the predetermined frequency. As
will be apparent, the quadrature feed of the microwave feeds causes
the field in the cavity to rotate with magnetic field loop 412
starting at the position shown in FIG. 15, and sequentially moving
to the positions shown in FIGS. 16, 17 and 18 with the time between
the "snapshots" shown in FIGS. 15-18 corresponding to successive
ninety electrical degrees incremental phase change between
successive Figures (also indicated by movement of magnetic field
loops 414, 416, 418 and 420 in the time succession shown). It is
also to be understood that the pattern of FIG. 15 will appear
ninety degrees after the time of the pattern shown in FIG. 18, with
the sequence repeating for as long as the magnetron is
operating.
The present invention has significant advantages over the prior
art. By using TM modes in the design process (especially where one
has the absence of a circumferential E field component to eliminate
edge overheating, particularly in "circular" loads such as pizza
and pita bread sandwiches) the present applicator increases cooking
efficiency (because TM type modes are better matched than TE type
modes to food type loads). The use of the selected TM modes, (where
the TM mode pair has degeneracy, i.e., the 2 times relationship of
guide wavelengths) in conjunction with a quadrature phase shift
feed structure creates an even, time-averaged energy distribution,
substantially eliminating hot and cold spots. The phase shift
structure of the present invention has no moving parts and is
therefore more mechanically efficient and reliable. Finally, the
applicator of the present invention offers increased safety by
minimizing microwave leakage.
In summary form, the procedure for determining dimensions of a
cylindrical cavity is as follows:
1. Choose a circular cylindric mode pair type for their rotational
symmetry which can be utilized for uniform heating and electronic
stirring.
2. Select only TM modes because of their characteristic high
coupling factor resulting in increased efficiency and low edge
overheating. Setting m=0 for a TM.sub.mn mode will result in a
pattern having an absence of an E field component in the
circumferential direction, which is advantageous for eliminating
edge overheating, but disadvantageous in that such a pattern (by
itself) will have undesirable "cold" regions. For example, the
TM.sub.2 mode will have a central "cold" spot and a concentric
annular "cold" ring shaped region. The second mode to be selected
is to have a "complementary" heating pattern to the first mode to
desirably "fill in" the "cold" spots or regions. For example a
TM.sub.11 mode will have a "hot" center region, and when rotated
will provide an even heating pattern without incurring edge
overheating.
3. Determine the free space wavelength for the microwave frequency
of interest (normally 2450 MHz) and determine the guide wavelengths
at that frequency for a range of diameters which encompass the
desired cavity diameter for the circularly symmetric TM mode types
previously selected.
4. Select the desired mode indices for the first mode to be used,
with the lower order mode indices (0 through 4) preferred since
they exhibit the most rapid change in guide wavelength as a
function of frequency, as indicated in FIG. 5; the TM.sub.02 mode
is preferred because it has circular symmetry in its magnetic field
and will provide strong heating at the peripheral region.
5. Select the desired mode indices for the second mode to be used,
where the second mode is a TM mode type, and has a guide wavelength
equal to one half the guide wavelength of the first mode selected,
at an acceptable cavity diameter. For example, at a diameter of
9.17465 inches, the TM.sub.02 mode has a guide wavelength of
12.55708 inches and the TM.sub.11 mode has a guide wavelength of
6.27854 inches.
6. For a resonant design, select the cavity height to be equal to
the guide wavelength of the first mode selected in step 4 above,
allowing the two chosen modes to be degenerate, i.e., to exist in
the same cavity at the same time, since the first mode will have a
half guide wavelength in the cavity vertically, while the second
mode will have a full guide wavelength field distribution
vertically in the cavity.
Once the dimensions of the cavity are determined as above, the feed
system can be determined according to the following additional
step:
7. Provide a quadrature feed system for the cavity wherein the feed
ports in the cavity are located in the top wall or in the side wall
at or near (i.e., <<.lambda..sub.g /4 for the shorter mode
guide wavelength) the top wall such that one feed port is located
90 angular degrees from the other feed port, as measured in a plane
perpendicular to the longitudinal axis; and provide an electrical
phase shift of 90 degrees from the one feed port to the other feed
port. It is to be understood that a positive or negative phase
shift may be used, with a resulting change in the direction of
rotation.
The invention is not to be taken as limited to all of the details
thereof as modifications and variations thereof may be made without
departing from the spirit or scope of the invention. For example
(but not by way of limitation), the load insertion may be by way of
an opening in the bottom wall with the shelf moving with the
closure of the opening. As another example, feed port spacings
other than 90 degrees (but with equal mechanical and electrical
angle values) are within the scope of the present invention. As a
still further example, it is within the scope of the present
invention to utilize an open-ended applicator where one wall, e.g.,
the bottom wall, is spaced apart from an adjacent wall, e.g., the
side wall, provided that means are included to block leakage from
between the side wall and the bottom wall.
* * * * *